Kinetic aspects of surfactant solubilization of soil-bound polycyclic aromatic hydrocarbons

The effect of nonionic polyoxyethylene (POE) surfactants on the solubilization rate of individual polycyclic aromatic hydrocarbons (PAHs) from a weathered, coal tar-contaminated soil obtained from a manufactured gas plant (MGP) site was studied. The release of PAHs upon surfactant washing were measured with time. Time-varying adsorption of PAHs to Tenax resin was also studied. Because of its large sorption coefficients for organic solutes, Tenax maintained near-zero concentrations of aqueous PAHs, thereby maximizing transfer of PAHs at the soil-water interface. The release of PAHs from the MGP soil exhibited a nonequilibrium behavior,typical of such soils. Surfactants significantly enhanced the rate of PAH solubilization, the amount solubilized being dependent upon the structure of the surfactant. It is postulated that the enhancement of PAH release by surfactants occurs by two mechanisms: (1) micellar solubilization by increased concentration gradient at the soil/tar-water interface and (2) sorption and penetration of surfactant molecules, causing intrasorbent swelling of the soil-tar matrix and increased matrix diffusivity of PAHs. A modified radial diffusion model was used to evaluate the relative importance of these mechanisms in surfactant washing of contaminated soils. It was concluded that surfactants enhanced PAH release from the test soil mainly by increasing matrix diffusivities, while increase in solubility by partitioning of PAHs into the micellar pseudophase played a secondary role.